The development of efficient light emitting organic compounds for the fabrication of optoelectronic and electroluminescent materials such as organic light emitting diodes (OLEDs), solar cells, and field-effect transistors (FETs) has commanded increasing attention in recent years. The development of fluorescent blue-light emitting organic compounds for the fabrication of electroluminescent materials has been fascinating, challenging and hot research object of academic and industrial endeavours.1 Various poly-acetylenes, poly-p-phenylenes, polyfluorenes, poly-heteroarenes and their cross-combination compounds with high luminescence quantum efficiencies have been developed.2-5 However, the practical applicability of these first-generation π-conjugated compounds in preparing electroluminescent devices is restricted by their tendency to form aggregates and exciplexes leading to emission band broadening and bathochromic shift, thus exhibiting low quantum yield in the solid state. To alleviate their tendency to aggregate, nonplanar and/or asymmetric π-conjugated three-dimensional molecular hierarchies have been suggested, which may reduce the fluorescence quenching resulting from orbital interactions/overlapping through spatial proximity of π-groups. For this purpose, several tailor-made multiple t-conjugated molecular architectures such as spiro-(spirofluorenes),6 ladder-(biphenalenes, phenylenes),7 propeller-(metal-quinoline complex such as Alq3),8 and double-decker-type (p-cyclophanes)9 monomers and polymers were synthesized.
Green Emission Defect:
Recently numerous tailor-made polyfluorenes or fluorenes with extended π-conjugated systems have proven their potential for preparing blue OLEDs with high quantum efficiencies10, the scope of their commercialization suffers from the appearance of additional undesirable low-energy ‘green emission’ band during operation, covering a broad range from 500 to 600 nm, which destroys the blue color purity.11 
The origin of ‘green emission’ band has been controversial and has not yet been fully understood.11d Initially it was believed that the origin of ‘green emission’ band was attributed to interchain aggregates and/or excimer formation, however no experimental observation11c supported the aggregates to be responsible for this low energy band. Instead, the experiments on fluorene-fluorenone systems suggest that the oxidation of fluorene to fluorenone is responsible for the emergence of this specific band. Researchers believe that such oxidation is possible either during polymerisation or by thermal-, photo-, or electrooxidation during device fabrication11c List et al11e proposed that highly active nickel (Ni0) species used in the reductive coupling of 2,7-dihalofluorene in the synthesis of poly-2,7-fluorenes may initiate oxidation of fluorene units to fluorenones. Holmes et al.11f recently demonstrated that it is possible to prepare oxidatively stable polyfluorenes by carefully prefixing the dialkyl substitution at position 9 of fluorenes.
At the moment, the most challenging topic of realization is to understand the parameters and problem of ‘green emission’ defect, which we presume may be addressed by identifying the agents that trigger the oxidation of fluorene to fluorenones. Alternatively we have come up with a new concept/approach to overcome this defect by shifting the green emission band to the blue region through appropriate functionalization of donor-acceptor moieties on fluorene and fluorenone and related scaffolds. In the present invention we have placed donor-acceptor substituents in such a way that donor acceptor fluorenones show emission in the blue region (instead of green-yellow region) thus improving the blue colour purity and overcoming the problem of green emission defect.
Synthesis of Fluorene Scaffolds:
Fluorenes contain a rigid biphenyl structure locked into a coplanar arrangement by the presence of a methylene moiety. In general, palladium-catalyzed Suzuki-Miyaura coupling protocol has been used to prepare a wide array of fluorene, spirofluorenes and related scaffolds12-14. The use of nickel as a catalyst in coupling reactions for the synthesis of polyfluorenes has also been reported.15 Despite the wide synthetic potential of these metal-assisted cross-coupling reactions, they suffer from the requirements for expensive organometallic reagents/catalysts, harsh reaction conditions, and undesired by-products. In addition, bulk production of these fluorenes for industrial purposes requires more investments for disposal of organometallic waste, purification of traces of metal impurities, and/or removal of by-products, from the final reaction mixture. Due to these limitations, in most of the reports, commercially available fluorene or 2,7-dihalofluorene has been used as a crucial precursor for preparing oligo- and polyfluorenes compromising with nonflexibility of introducing donor-acceptor substituents in their molecular scaffolds.16 Therefore, developing simple, fast and general synthetic routes for fluorene and fluorenone structures is highly essential to further expanding the scope of applications of these rigid systems.
The present invention relates to a highly rapid novel synthesis of a new series of donor-acceptor fluorenes, fluorenones and their t-conjugated systems.
Oxidation of Fluorenes to Fluorenones:
Literature methodologies for the direct oxidation of fluorene to 9-fluorenone require specialized homogeneous or heterogeneous catalysts or harsh reaction conditions.17 The present invention also relates to new highly rapid method for the oxidation of unsubstituted or substituted fluorenes to corresponding fluorenones by aerial oxidation without using any catalyst in the presence of a base such as metal hydrides or alkaline earth metal hydrides in an appropriate solvent such as THF at the temperature ranges from −30° C. to 25° C.
Some of related references and patents based on the present invention are mentioned below: